1. Field of the Invention
The present invention relates generally to radio communication technologies, and more particularly, to a method and apparatus for generating a Dedicated Reference Signal (DRS) in a radio communication system.
2. Description of the Related Art
In an Advanced Long Term Evolution (LTE-A) system, 8 transmitting antennae are configured for each cell to support a higher peak rate. In order to decrease the overhead of Reference Signals (RS), a DRS is used to demodulate downlink data. A base station sends a DRS for each stream of data of each User Equipment (UE). Most UEs in a cell adopt a low rank transmission mode, wherein the value of the rank is equal to the number of streams simultaneously transmitted by the UE. As such, the number of DRSs actually transmitted by each UE is small, thereby decreasing the overhead of reference signals. In addition, the use of the DRS is convenient for Coordinated Multi-Point (CoMP) transmission and Multi-User Multiple-Input Multiple-Output (MU-MIMO) transmission.
In the LTE-A system, the CoMP transmission is mainly used for improving the average throughput of a cell and the throughput of a cell boundary, and includes two specific implementation modes, i.e. coordinated scheduling and coordinated multi-point joint transmission. For the coordinated scheduling, data of one UE only comes from one transmitting node, i.e. one serving cell. Data transmitted by other nodes is received as interference, and thus multiple nodes need coordinated scheduling to control an interference level. For the coordinated multi-point joint transmission, multiple nodes may transmit data to one UE by using the same time-frequency resources, thereby increasing the signal-to-noise ratio of the UE and decreasing the interference. Based on a DRS, the LTE-A system may support transparent coordinated multi-point joint transmission. The UE only receives data of a Physical Downlink Control CHannel (PDCCH) transmitted by one node, and the node is a serving cell of the UE. At the same time, data transmission is based on the DRS, and the UE receives the DRS, performs channel evaluation and demodulates the data, but does not consider which nodes transmit the data.
In addition, since MU-MIMO is adopted in the LTE-A system, data of multiple UEs can be transmitted by using the same time-frequency resources. The MU-MIMO has also been supported in a Long Term Evolution (LTE) system. However, MU-MIMO in the LTE solution is defined based on Single-User MIMO (SU-MIMO), which limits performance gain. In the LTE-A system, the MU-MIMO is to be optimized, by providing scheduling information of other UEs performing MU-MIMO with one UE, so that the UE can report more accurate Channel Quality Indicator (CQI) information, and remove interference as much as possible when demodulating data. For MU-MIMO based on a DRS, a base station is configured such that multiple UE performing MU-MIMO adopt different DRS patterns that preferably are orthogonal, to obtain a better channel evaluation performance.
FIG. 1 illustrates a conventional DRS structure. A Code Division Multiplexing (CDM) and Frequency Division Multiplexing (FDM) mode is adopted, i.e. the Resource Elements (RE) for transmitting DRSs are divided into two groups by using a FDM mode, and DRSs of multiple data streams are multiplexed in each RE group by using a CDM mode. Hereinafter, each group of REs will be referred to as a CDM RE group.
FIG. 2 illustrates a conventional method for generating and mapping a DRS. As shown in FIG. 2, an RS sequence of each antenna port, i.e. a DRS sequence, is generated, and then RE mapping is performed for the RS sequence. There are three methods for generating the RS sequence. In the first method, one random sequence is generated for each antenna port by using different initialization values, and then the RS sequence is generated according to the random sequence. In the second method, one random sequence is generated by using one initialization value, one long scrambling sequence is generated according to the random sequence, and the long to scrambling sequence is divided into multiple subsections and the RS sequence of each antenna port is obtained. In the third method, one random sequence is generated by using one initialization value, then one scrambling code sequence is generated according to the random sequence, and each antenna port uses the scrambling code sequence as the RS sequence.
For a DRS structure using the CDM mode or a DRS structure jointly using the CDM mode and other multiplexing modes, the step of generating the RS sequence shown in FIG. 2 can be described in detail, e.g. a method for generating and mapping a DRS shown in FIG. 3. As shown in FIG. 3, an RS sequence of each antenna port is generated, then each element of each RS sequence is spread to obtain an spread RS sequence, where an spreading code may be a Walsh code, and RE mapping is performed for the spread RS sequence.
For the method shown in FIG. 3, if RS sequences of all antenna ports are identical, when DRSs of multiple data streams are transmitted by using the CDM mode, it is possible for DRSs on one OFDM symbol to adopt double transmission power and DRSs on the other OFDM symbol to have no transmission power, thereby influencing total power for data transmission. If the RS sequences of all antenna ports are independent from each other, an imbalance of transmission power of DRSs is not a problem. However, the method shown in FIG. 3 cannot restrain DRS interference between different cells. Generally, the smaller the interference, the better the performance, but the method shown in FIG. 3 results in the interference fixedly coming from a certain cell or UE.